The present invention relates to a hot dip zinc-coated steel sheet having excellent tensile strength and ductility balance and excellent coating adhesion, which sufficiently endures a complicated press molding forming, and a method of producing the same.
The hot dip zinc-galvanized steel sheet of the present invention includes that which contains alloy elements such as Fe in the zinc coating layer thereof.
In general, hot-rolled steel sheet and cold-rolled steel sheet exhibit poorer ductility (i.e., poorer total elongation, bending property and the like) as the tensile strength thereof increases, whereby complicated press forming of the steel plate becomes difficult to perform.
It is also generally known that adding elements such as Mn, Si and the like for the purpose of increasing the tensile strength of the steel sheet solid solution strengthing and achieves an excellent composite-structure, which is advantageous for improving the balance between the tensile strength and elogation.
However, as Mn, Si and the like are elements which are easily oxidized, if such elements are added by a large amount, Si, Mn and the like segregated on the surface of the steel sheet during annealing, whereby the wetting property of the steel sheet with respect to molten zinc deteriorates, resulting in significantly poor reactivity between the base steel and the molten zinc.
In this case, the coating adhesion property deteriorates due to such poor reactivity, and exfoliation of coating what is called xe2x80x9cpoweringxe2x80x9d or xe2x80x9cflakingxe2x80x9d is generated during forming.
As a method of solving the aforementioned problems and producing a high tensile strength hot dip zinc-galvanized steel sheet having excellent formability, JP-A 5-179356 and JP-A 5-51647, Laid-Open, for example, disclose a method of: rapidly quenching-cooling the steel sheet during the hot rolling coiling process; annealing the steel sheet in the dual phase region in a hot dip Zn galvanizing line; and carrying out galvanizing.
However, in practice, if Si is added even by a very little amount, there arises a problem that the coating adhesion deteriorates and coating exfoliation of coating is likely to occur.
Therefore, it has conventionally been considered that production of a high tensile strength hot dip Zn galvanized steel sheet having excellent coating adhesion is impossible when a steel sheet containing relatively large amounts of Si, Mn and the like is used as the base material.
Further, the inventions of (1) PCT/JP99/04385; (2) PCT/JP97/00045; and (3) PCT/JP00/00975 disclose, respectively: (1) a coating method of a high tensile strength steel sheet containing Mo; (2) a coated steel sheet having an oxide layer in the base steel surface layer portion of a steel sheet; and (3) a coated steel sheet having an oxide layer produced by annealing a base steel having mill scale on the surface.
According to the invention (1) described above, a coated sheet having high tensile strength and excellent coating adhesion can be obtained. However, as the micro structure of the base material is not subjected to sufficient control, the desired ductility which is required in addition to the tensile strength cannot be obtained. Further, as the inner oxide layer is not subjected to any control, the resulting product of the invention (1) can only insufficiently meet the strict requirements, which have been demanded in recent years and are necessary for the present invention, for the balance between tensile strength and ductility and the coating adhesion.
In the invention (2) described above, high tensile strength can be obtained by appropriately selecting chemical compositions of the steel, and the resulting coating adhesion is also excellent. However, as the structure of the base material is not subjected to any control, as is the case in the aforementioned invention (1), the desirable ductility which is required in addition to the tensile strength cannot be obtained in the invention (2), either. Accordingly, the steel sheet of the invention (2) can only insufficiently meet the requirements in performances which are necessary in the present invention.
Further, in terms of quality control of coating, requirements in the coating adhesion property are now becoming more strict than before, due to more varied applications of a high tensile strength steel sheet, which is combined with the significant increase in use of such a steel sheet in recent years. Accordingly, it is becoming harder to satisfy the requirements in the coating adhesion property as described above by simply forming an internal oxide layer.
More specifically, such strict requirements as described above cannot be satisfied unless the compositions of the base steel right under the coating layer are also controlled, as disclosed in the present invention.
Further, in the invention (3) described above, high tensile strength is obtained by appropriately selecting the compositions of the steel, similarly to the aforementioned invention (2). However, as the structure of the base material is not subjected to any control, as is the case in the aforementioned invention (1), the desirable ductility which is required in addition to the tensile strength cannot be obtained in the invention (3), either. Accordingly, the steel sheet of the invention (3) can only insufficiently meet the requirements in performances which are necessary in the present invention.
The requirements for the coating adhesion property are now becoming more strict than before, as explained with respect to the invention (2). Such strict requirements cannot be satisfied in the invention (3), either, unless the compositions of the base steel right under the coating layer are also controlled as disclosed in the present invention.
The present invention has an object to solve the aforementioned problems of the prior art and provide, when the base steel sheet (i.e., the base steel) contains relatively large amounts of Si, Mn and the like, a high tensile strength hot dip Zn galvanized steel sheet having excellent coating adhesion and ductility, that is, a hot dip Zn galvanized steel sheet having excellent balance between tensile strength and ductility and excellent coating adhesion.
In addition, the present invention has another object to provide a method of advantageously producing the hot dip Zn galvanized steel sheet exhibiting excellent performances as described above.
Specifically, the structure of the present invention can be summarized as follows.
1. A hot dip Zn galvanized steel sheet having excellent balance between tensile strength and ductility and excellent coating adhesion, an average composition of a base steel thereof comprising:
0.05-0.25 mass % of C;
not more than 2.0 mass % of Si;
1.0-2.5 mass % of Mn; and
0.005-0.10 mass % of Al,
xe2x80x83wherein the C content at the base steel surface layer portion right under a coating layer is not more than 0.02 mass %, the base steel structure contains martensite phase, by not less than 50% as a fraction percentage, the martensite phase including both tempered martensite phase and fine size martensite phase, and the remaining portion of the base steel structure being formed by ferrite phase and residual austenite phase.
2. A hot dip Zn galvanized steel sheet having excellent balance between tensile strength and ductility and excellent coating adhesion of the aforementioned (1), wherein at least one type of oxide selected from the group consisting of Si oxides, Mn oxides, Fe oxides and composite oxides thereof is present, in at least one of grain boundary and crystal grain of a region in which the C content is not more than 0.02 mass % of the base steel surface layer portion right under the coating layer, and the amount of oxides generated on the base steel surface layer portion, when it is converted into the amount of oxygen, is 1-200 mass-ppm.
3. A hot dip Zn galvanized steel sheet having excellent balance between tensile strength and ductility and excellent coating adhesion of the aforementioned 1 or 2, wherein the Fe content of the coating layer is in the range of 8-12 mass %.
4. A method of producing a hot dip Zn galvanized steel sheet having excellent balance between tensile strength and ductility and excellent coating adhesion, comprising the steps of:
preparing hot-rolled steel sheet or cold-rolled steel sheet having a steel sheet average composition which includes: 0.05-0.25 mass % of C; not more than 2.0 mass % of Si; 1.0-2.5 mass % of Mn; and 0.005-0.10 mass % of Al;
heating the hot-rolled steel sheet or cold-rolled steel sheet in an atmosphere which satisfies the following formula at a temperature of 800-1000xc2x0 C.:
log(H2O/H2)xe2x89xa72.5[C]xe2x88x923.5
wherein H2O/H2 represents the ratio of the partial pressure of moisture in the atmosphere with respect to the partial pressure of hydrogen gas, and [C] represents the amount of C in the steel (mass %).
5. A method of producing a hot dip Zn galvanized steel sheet having excellent balance between tensile strength and ductility and excellent coating adhesion of the aforementioned 4, further comprising the step of subjecting the steel sheet to galvannealing alloy process at a temperature of 450-550xc2x0 C. after completing the hot dip Zn galvanizing process.